The present invention relates to a method and a system for steering a wheeled vehicle, and more particularly a method and a system for steering a wheeled vehicle of the type having a pair of rear steerable road wheels, a pair of front steerable road wheels, and a steering wheel, wherein the angle through which the pair of rear steerable road wheels is steered is in a functional relationship to the angle through which the pair of front steerable road wheels is steered for controlling a steering force applied to the pair of front steering road wheels.
In ordinary automotive vehicles, only the front road wheels or the rear road wheels are steerable. Some vehicles, such as those having exceptionally elongated wheel bases, use two or more pairs of steerable road wheels. These pairs of steerable road wheels are coupled together either mechanically or through hydraulic circuits. One pair of steerable road wheels is steered by human intervention and another pair of steerable road wheels is steered through an angle proportional to the angle through which the former pair of road wheels is steered. At low speeds, this angle is in the opposite direction of the angle of the former wheels. As speed increases, the angle decreases until the polarity of this angle matches that of the former wheels. A vehicle having such a set of steerable road wheels is capable of being steered through a sharp turn at a low speed where ample space is not available for maneuvering the vehicle.
When a vehicle of this type is making a turn, the vehicle is subject to not only yawing motion but side slip about a vertical axis which passes through the center of gravity of the vehicle. Due to the side slip thus involved in a turning maneuver of the vehicle, the vehicle is not capable of maintaining travel in a direction tangential to the turning circle so that the vehicle driver does not have complete control over the direction in which the vehicle is to advance.
Since, furthermore, the side acceleration of the vehicle as sensed by the vehicle driver does not conform to the centrifugal acceleration of the vehicle making a turn, the vehicle driver is compelled to maneuver the vehicle with a strained sensation.
The present invention contemplates elimination of these drawbacks thus far encountered in a vehicle using two or more pairs of steerable road wheels.
According to this method, however, as the vehicle speed increases, the yawing rate gain, defined as the yawing rate divided by the front wheel steering angle, decreases. If the vehicle is travelling on a level surface at a speed V (meters per second), the theoretical relationship between the yawing rate r (radians per second) and radius of the turning circle R (meters) can be shown to be: EQU R=V/r
Consequently, when the speed of the vehicle increases but the yawing rate is not increased, the radius of the turning circle cannot be kept constant but tends to increase, causing apparent under-steering characteristics to develop.
As is obvious from above, when the rear wheels are steered so as to make the side-slip angle small, with increasing vehicle speed, apparent high severe understeering is developed. Consider the case of vehicles at different speeds on a bend on a high-speed road. While intending to turn in a circle of constant radius while steering at a constant steering angle, the bad situation is created in which as the speed gradually increases, the rear wheels move into alignment with the front wheels and the radius of the turning circle increases. Consequently, if the speed drops due to braking while turning and maintaining a constant steering angle, the rear wheels turn so as to be in a different direction from the front wheels, and an undesirable condition known as "tack-in" will occur in which the vehicle is misaligned with the direction of travel or similar phenomena. The present invention also eliminates these drawbacks.